In order to determine the wettability of rocks with respect to the water and the oil which may be contained therein, the initially water-saturated rock is, for example, subjected to a drainage process in order to decrease the saturation (Sw) thereof, then to an imbibition process allowing the water saturation thereof to be increased again. The capillary pressure at one point is defined as the difference Pc at equilibrium between the pressure Po of the oil and the pressure Pw of the water. This parameter makes sense only if the two fluids are in the continuous phase in the porous medium. For a water wet medium, only positive values make sense. On the other hand, if the medium has a mixed wettability, the fluids can remain in the continuous phase for the positive and for the negative capillary pressures (Pc) as well.
For an application of this type, a complete capillary pressure measuring cycle must therefore comprise (FIG. 4):
a) positive primary drainage of an initially 100% water-saturated sample (curve C1); PA1 b) positive imbibition (curve C2); PA1 c) negative imbibition (curve C3); PA1 d) negative drainage (curve C4); and PA1 e) positive secondary drainage (curve C5).
Curves C2' and C3', respectively, correspond to a spontaneous imbibition and a negative primary imbibition.
Patent application EP-A-0,701,120 filed by the claimant describes a device intended for wettability measurements on porous rock samples in a containment cell delimited at the two opposite ends thereof by two end pieces. Channels run through these end pieces and communicate the grooved internal faces thereof respectively with two adjustable pressure fluid sources delivering, respectively, a first and a second fluid. The bar is placed in the cell within an elongate deformable sheath associated with pressure means intended to laterally exert a confining pressure, thereon. Two semipermeable membranes, respectively, permeable to the first and to the second fluid are placed between the internal faces of the end pieces and the sample. Wraparound electrodes are placed in the sheath, in contact with the lateral wall of the sample, on either side thereof. They are connected to a device measuring the electrical conductivity of the sample. The bar is at first initially saturated with a first fluid, such as brine, prior to being placed in the cell. A second fluid, oil for example, is injected under pressure at one end of the cell. It displaces the first fluid in the sample and the volume ejected is recovered at the opposite end. One or more of the next phases of the aforementioned measuring cycle are carried out thereafter.
A measuring device comprising means for measuring the electrical conductivity and the volume of the fluid ejected is placed in the cell, thus allowing to determine various physical parameters of the bar.
In a structure of this type, it is well-known to use as an oil wet membrane, a membrane for example made of Gore-Tex (registered trademark) which withstands the temperature range (up to 100 or 200.degree. C.) observed during porous sample tests.
A thin plate made of porous ceramic, which fills its role as long as the operating temperature of the cell is not too high (preferably below 100.degree. C.), is generally used as a water wet membrane. It is possible to use a ceramic membrane that is a few mm thick and which withstands the temperature better, but the drawback of such a membrane is that it notably slows the progress of the cycle of operations down.